A motor driven brake-by-wire system typically operates in one of two braking modes: 1) normal braking; or 2) emergency intervention braking. In normal braking, the vehicle's brake system controller outputs slowly modulated brake commands to a brake-by-wire motor controller in direct response to driver-generated input signals from a brake-pedal force and brake travel sensor. During normal braking mode, the motor's output torque efficiency is maximized to enable the motor to output moderate torque levels over relatively long periods of time at low power consumption rates.
In contrast, a brake-by-wire motor controller system in emergency intervention braking mode responds to an output torque command from the vehicle's brake system controller based upon a variety of vehicle sensor inputs. The emergency intervention braking mode of pulsing the brake caliper to avoid tire skid or wheel lock during ABS, operation, for example, is characterized by large output torque demands from the brake-by-wire motor applied over relatively short time intervals. During emergency intervention braking mode, lower motor output torque efficiency is accepted as a trade-off to improve the brake-by-wire system's ability to closely track variable torque output commands under adverse conditions such as sudden fluctuations in motor supply voltage and high coil temperatures.
An ideal control system for a SRM driving a brake-by-wire system would enable the SRM to deliver output torque dynamically despite variations in multiple system parameters such as motor speed, motor supply voltage, and motor coil temperature. To accomplish this goal, a simplified method is needed for determining the optimum angular position of the SRM's rotor at which to energize and de-energize the phase windings of the stator, so called "turn-on" and "turn-off" angles or, collectively, the "conduction angles." Various complex schemes exist for selecting the optimal conduction angles that maximizes the SRM's output torque and operating efficiency despite variation in its instantaneous operating speed. The present invention provides a system that simplifies the determination of optimal conduction angles that compensate for variations in the motor's rotor speed as well as variations in the motor's supply voltage and operating temperature that arise as a result of changing load conditions and coil resistance.
Traditionally, selection of an optimal turn-on angle can be made independently from the selection of an optimal turn-off angle only for the case in which the motor is operating at low speed using "flat-top" current control. Flat-top current control is characterized by a phase current vs. time function in which no significant time delay exists between the instant in time that the phase current (i.e., the current that runs through the stator's phase windings) is turned-on and the instant it reaches its maximum phase current value. Similarly, no significant time delay exists between the instant in time that the phase current is turned-off and when it reaches a zero value.
More complicated phase current control schemes that determine conduction angles that optimize the motor's output torque and efficiency for cases of higher and/or variable speed motor operation typically employ lengthy look-up tables that require the processing power of a micro-controller to access. The highly coupled relationship between the turn-on and turn-off angles in traditional high speed motor control schemes complicates their determination. If additional system conditions or parameters such as variations in the motor's coil resistance and the motor's supply voltage are considered, determination of optimal turn-on and turn-off angles is further complicated. The present invention provides a simplified system for determining optimized conduction angles for SRMs operating under varying conditions.